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Drop a hole that no ring can parent instead of failing the run (#401)

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* wagyu: drop a hole no remaining ring can parent instead of throwing

correct_tree() throws "Could not properly place hole to a parent" when
topology correction leaves a hole whose parent ring was removed (degenerate
input such as stacked duplicate rings from coalesced tiny-polygon
placeholders). That aborts the entire tiling run over one unrepresentable
sliver. Remove the ring and its points instead, matching how other
unresolvable degeneracies are handled.

* Add a regression test for dropping an unplaceable hole

A fuzzer-minimized pair of mutually reversed self-intersecting rings that
makes wagyu's correct_tree fail to find a parent for a hole — the same
failure reported in mapbox/tippecanoe#761. Before the topology_correction
change, running this test exits with EXIT_IMPOSSIBLE via the polygon
cleaning error handler; with it, the clean returns.
This commit is contained in:
Brandon Keepers
2026-08-05 08:45:39 -07:00
committed by GitHub
parent 7c80fccdc1
commit 1b060b7faf
2 changed files with 37 additions and 1 deletions
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unit.cpp
+33
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@@ -136,3 +136,36 @@ TEST_CASE("line_is_too_small") {
dv.emplace_back(VT_LINETO, -51864809, 2683873977);
REQUIRE(line_is_too_small(dv, 0, 10));
}
TEST_CASE("Polygon cleaning drops a hole that no ring can parent", "[wagyu]") {
// Two mutually reversed self-intersecting rings whose union leaves a hole
// that wagyu's topology correction cannot assign to any surviving parent
// ring (found by fuzzing; same failure as mapbox/tippecanoe#761). Without
// the fix in mapbox/geometry/wagyu/topology_correction.hpp, this exits
// through the "Could not properly place hole to a parent." handler in
// clean_or_clip_poly instead of returning.
static const std::vector<std::vector<std::pair<long long, long long>>> rings = {
{{0, 5}, {5, 4}, {5, 1}, {4, 4}, {4, 2}, {7, 1}, {0, 5}},
{{0, 5}, {7, 1}, {4, 2}, {4, 4}, {5, 1}, {5, 4}, {0, 0}, {0, 5}},
};
drawvec geom;
for (auto const &ring : rings) {
for (size_t i = 0; i < ring.size(); i++) {
geom.push_back(draw(i == 0 ? VT_MOVETO : VT_LINETO, ring[i].first, ring[i].second));
}
}
drawvec out = clean_or_clip_poly(geom, 0, 0, false, false);
// The regression signal is getting here at all: without the fix,
// clean_or_clip_poly exits the process from its wagyu error handler.
SUCCEED("clean_or_clip_poly returned");
// Anything that survives must be sanely wound: first ring positive.
if (out.size() > 0) {
size_t j = 1;
while (j < out.size() && out[j].op == VT_LINETO) j++;
REQUIRE(get_area(out, 0, j) > 0);
}
}